The episode “The Tenth Planet” marks an important turning point in the history of Doctor Who – and not only because it introduced the Cybermen who would feature perennially throughout the show’s history. This was the last episode with William Hartnell. Doctor Who seems to be rather unique in having discovered a way to change the actor playing the show’s central character and offer an explanation in terms of the show’s mythology, rather than trying to pretend that nothing is different. This more than anything else must be the secret to the show’s longevity, and whatever one’s feelings about Doctor Who, it is surely appropriate to admit that it is a brilliant idea and one that has proven highly successful.

The episode “The Tenth Planet” begins with a concept which is echoed in the recent movie (which I have not seen) Another Earth. A new planet, with land masses resembling Earth’s, is detected in the solar system. The Doctor seems to know quite a bit about what is going on, saying that Earth had a twin a long time before, called Mondas.